1. Field of the Invention
The present invention relates generally to a dynamic aperture for adjusting the amount of light of a projection lens in a projection television or projector and, more particularly, to an apparatus for driving a dynamic aperture and a method of controlling the same that can be implemented using a Hall sensor to have a simple configuration and can perform precise position control regardless of variation in ambient temperature.
2. Description of the Related Art
Recently, large screen and high definition display devices attract attention as one of important issues, representatives of which are projection televisions (TVs) and projectors.
Such projection TVs and projectors are operated according to optical principles, and FIG. 1 shows an example of the optical configuration of such a projection television or projector.
Referring to FIG. 1, a projection apparatus generally includes an illumination optical system 1a for generating light, a reflective display device 1b for imposing an image on the light incident from the illumination optical system 1a by reflecting the light on a pixel basis according to a provided image, and a projection optical system 1c for projecting the image, which is reflected through the reflective display device 1b, on a screen.
The illumination optical system 1a includes a light source 10 having a lamp for generating light and a reflecting mirror for reflecting light to guide the light along a propagation path, and an optical lens 20 for irradiating the light, which is emitted from the light source 10, onto the reflective display device 1b. 
The optical lens 20 includes a condensing lens 21 for condensing light, which is emitted from the light source 10, onto the display device 1b and a shaping lens 23 for shaping the condensed light while converting the condensed light into collimated light. In this case, a color wheel 22, which is rotated in a single direction by a driving device (not shown) and has a plurality of color filters arranged along a radial direction at regular intervals, is placed between the condensing lens 21 and the shaping lens 23 to selectively transmit desired color light during the rotation thereof.
The display device 1b is implemented using a digital micromirror device (DMD) 30, which is mounted on a substrate 33 having a processor 31 and memory 32 and separates the optical path of the illumination optical system 1a and the optical path of the projection optical system 1c according to the tilt angle thereof.
The DMD 30 is a projection type display device that was developed by Texas Instruments Inc. and employs an optical semiconductor for controlling light. The DMD 30 is provided with a plurality of micro reflecting mirrors having a micro size that are two-dimensionally arranged on a silicon wafer. Each of the reflective micro mirrors handles and corresponds to a pixel structure, and the tilt of the mirror is adjusted by a corresponding electrostatic system of the memory 32, which is arranged to correspond to each pixel, thus implementing the image. Each of the reflecting mirrors of the DMD 30 reflects incident light while switching the optical path of the light between two states (ON/OFF) by the tilting movement thereof at a high speed of 10 μs.
In other words, when the reflecting mirror is tilted and switched to the ON state, the light reflected by the reflecting mirror is enlarged through the projection lens module 40 of the projection optical system 1c and is then irradiated onto a screen 50. In contrast, when the reflecting mirror is tilted and switched to the OFF state, the light incident on the reflecting mirror cannot be irradiated onto the screen 50. The DMD 30 imposes image information on light provided by the illumination optical system 1a by selectively turning on and off the light in such a way as to individually tilt the reflecting mirrors according to image signals for individual pixels and, therefore, vary the reflecting angles of the individual mirrors.
The projection optical system 1c is formed of the projection lens module 40, and focuses an image, which is transmitted from the DMD 30, on the screen 50 while enlarging the image.
In this case, the projection lens module 40, as shown in FIG. 2, is formed of a plurality of projection lenses which are sequentially arranged along an optical axis in a lens barrel 41 and have predetermined individual diameters and individual optical properties so that the image incident from the DVD 30 is enlarged to be clearly focused on the screen 50 that is spaced apart therefrom by a predetermined distance. In this case, the focal distance of the projected image can be adjusted by controlling the interval between two adjacent projection lenses 42.
Furthermore, an aperture 43 is provided between the projection lenses 42 to adjust the amount of light so that a projected image has appropriate contrast. In this case, in order to precisely adjust the contrast, the position of the aperture 43 must be controlled at 128 high resolution steps within an arbitrary angle (e.g., 30 degrees), and a Voice Coil Motor (VCM) 44 is generally used to rotate the aperture 43 so as to perform precise position control.
FIG. 3 shows a conventional structure for controlling the position of an aperture. A conventional apparatus for controlling an aperture 110 includes a pivot 111 integrated with the aperture and adapted to be rotated to the right and left, a sensor magnet 112 adapted to provide different magnetic intensity according to the rotating angle of the pivot 111, a Hall sensor 113 integrally rotated along with the pivot 111 and adapted to convert the intensity of a magnetic field, which is provided by the sensor magnet 112, into an electric signal, a stopper 114 located at a limit position of the rotational range of the pivot 111 to stop the pivot 111 which tends to be rotated over the limit position, a driving magnet 115 located on the rotational path of the lower end of the pivot 111, and a driving coil 116 installed on the lower end of the pivot 111 to be opposite to the driving magnet 115 to rotate the pivot 111 according to driving current generated by electromagnetic interaction with the driving magnet 115. The driving magnet 115 and the driving coil 116 correspond to the VCM.
The aperture driving apparatus performs feedback control to cause the pivot 111 to move to a designated position in such a way as to rotate the pivot 111a by applying current to the driving coil 116 and detect the position of the pivot 111 using the sensor magnet 112 and the Hall sensor 113.
A conventional process of detecting the position of an aperture is described below. As shown in FIG. 4a, in the initial state in which current is not applied to the driving coil 115, the pivot 111 is stopped by the stopper 114 and the output of the Hall sensor 113 is then stored as a reference value. Thereafter, when the pivot 111 is rotated by a certain angle as shown in FIG. 4b, the output of the Hall sensor 113 is changed and the rotating angle of the pivot 111 is estimated from the difference between the stored reference value and the output of the Hall sensor 113.
However, when ambient temperature varies in the case where the position of the pivot 111 is detected using the Hall sensor 113 as described above, the magnetization of the sensor magnet 112 corresponding to the rotating angle of the pivot 111 is changed, so that the output of the Hall sensor 113 is changed by the ambient temperature even though the pivot 111 is located at the same position. As a result, since the position feedback value is changed depending on the ambient temperature, the reliability of the driving of the aperture is deteriorated.
Furthermore, instead of the Hall sensor, an optical sensor or MR encoder which is not subject to the influence of ambient temperature may be used to perform the position control, which results in high cost and complex circuit configuration.